The amount of electromagnetic radiation (e.g., light) that a camera photosensor collects and images may depend on the location of objects in the field of view (FOV) of the camera. Briefly, an object that is on the camera's optical axis may appear brighter to the camera's photosensor than an object that is off-axis, other factors being equal. The term “irradiance” generally refers to the electromagnetic radiation that is incident on a surface, such as a photosensor. The terms “radiant exitance” or “radiant emittance” generally refer to electromagnetic radiation that emerges (e.g., reflects) from an object. Irradiance, as well as exitance may be measured per second per unit area. For an object having an angular displacement “θ” relative to the camera's optical axis, the irradiance may, in general, decrease as cos4 θ.
For various applications, it can be advantageous for objects in a scene imaged by a camera to have substantially the same irradiance on the camera photosensor independent of the angular displacement θ. For example, a time of flight (TOF) three-dimensional (3D) camera determines distances to objects in a scene that the camera images by timing how long it takes for light transmitted by the camera to travel to the objects and back to the camera. For some systems, an illumination system transmits the light to the object in very short light pulses. The camera images light that is reflected by the objects and is collected by the photosensor to determine round trip travel times for light. Accurate distance measurement depends on the irradiance associated with the image of the object captured by the photosensor. Moreover, accuracy may improve with increase in the irradiance.